Adds BlurMaterial + blur.wgsl with a 13-tap weighted Gaussian kernel
(var<private> arrays — naga rejects dynamic indexing of const arrays),
run in downsample then upsample passes. For the 3-level pyramid:
bloom_0 (half, from brightpass) → bloom_1 (quarter) → bloom_2 (eighth),
then upsample to bloom_final (half). Camera orders -18/-17/-16/-15.
The bloom output is not yet composited (Task 6); this commit only wires
the passes and confirms they run in sequence without crashing.
